A major problem encountered in injection-molding plastic materials involves removal of moisture and other volatiles contained in these materials, prior to injection into a mold. Failure to adequately remove moisture from the materials tends to produce air pockets in the resultant product and to seriously degrade the quality of the product. It is consequently a common practice to dry such materials in a reservoir through which a flow of heated air is passed. Several hours of drying are usually required, and a considerable amount of energy is expended during the drying process. To conserve energy, warm, damp air exhausted from the reservoir is sometimes circulated through a dessicant filter, re-heated and then re-circulated through the reservoir. The filters must periodically be purged of water, and elaborate systems have been proposed for removing filters between an operative, in-line position and purging apparatus, without significantly interrupting the drying process.
Another method of removing moisture prior to actual injection involves venting the barrel of the molding machine. A problem faced in venting the barrel is the possibility that internal pressure will force plasticized materials into the vent. An injection molding machine adapted for barrel venting is described in U.S. Pat. No. 3,826,477 which issued on July 30, 1974 citing Mahito Kunoti et al as inventors. The Kunoti patent describes an injection molding machine in which the barrel is formed with sections of stepped internal diameter, and in which the screw thread is adapted to reduce the accumulation of plasticized materials in a particular region of relatively wider diameter. The vent is then introduced into this region where pressure is significantly reduced to permit the venting of volatiles without significant expulsion of plasticized material into the vent. Prior techniques of venting a stepped barrel have not been entirely successful, and in some instances a measure of pre-drying has been felt necessary to obtain a molded product of satisfactory quality.
The invention provides a device and associated method for introducing particulate plastic materials into a machine of the injection-molding or extrusion type which completely eliminates the need for pre-drying and which removes moisture and other volatiles in a simple, yet effective manner that does not require modification of an existing machine or construction of a machine with a stepped barrel or complex screw-drive.